1. Field of the Invention
This invention relates to a photosensor.
2. Related Background Art
To detect illuminance of light of the surrounding environment, photodiodes or phototransistors made by using simplex Si (silicon) have been used as detectors. Detectors are often used to detect visible light in the wavelength band approximately from 400 nm to 720 nm and to control devices. For example, detectors are often used in liquid crystal monitors of electric devices such as portable phones, mobile computers, and so forth. In such a device, if the environment is bright for human eyes, the detector detects the illuminance of the ambient visible light, and the electric device automatically controls and darkens the backlight of the liquid crystal to minimize consumption of the battery.
FIG. 10 shows graphs of spectral responses of a Si phototransistor, standard eye responses, and wavelength characteristics of various light sources. The graph a shows spectral responses of a Si-phototransistor, and the graph b shows the standard eye responses. The graphs c, d and e show wavelength characteristics of sunlight, fluorescent light, and incandescent light used as light sources, respectively.
The graphs are shown as relative sensitivities and relative intensities with respect to wavelengths of light waves. Relative sensitivities and relative intensities are ratios of sensitivity values or intensity values relative to maximum values of sensitivities or intensities.
Standard eye responses represent responses of visible light to eyes. For normal human eyes, light having the wavelength around 555 nm is most visible. Therefore, standard eye responses are maximized around 555 nm, and have an extension as shown by the graph b within the wavelength band of visible light approximately from 400 nm to 720 nm.
However, as shown in FIG. 10, sunlight has a small part of its intensity in the infrared band, and light of an incandescent light has a large part of its intensity in the infrared band (see the graphs c and d).
Additionally, in spectral responses of the Si phototransistor, sensitivity to light of the infrared band is maximum (see the graph a). That is, the Si phototransistor has a relatively low spectral response to visible light and a relatively high spectral response to infrared light.
Therefore, even in an environment where the illuminance of visible light is relatively low and it is dark for human eyes, the Si phototransistor may undesirably detect and react on sunlight, or especially infrared light from an incandescent light. This invites malfunctions of the electric device. For example, even when the environment is dark for human eyes, the electric device nevertheless darkens the backlight of its liquid crystal. Therefore, to ensure the detector reacts only on the illuminance by visible light, it is necessary to adjust the spectral responses of the detector nearer to the standard eye responses.
To ensure a Si phototransistor accurately detects the illuminance of visible light exclusively, the existing techniques locate a correction luminous-efficiency filter outside the Si phototransistor. The luminous-efficiency correction filter adjusts the spectral responses of the Si phototransistor (see the graph a) nearer to the standard eye responses (graph b) by correcting the wavelength of light entering into the Si phototransistor.
However, the use of the luminous-efficiency correction filter inevitably increases the size of the detector and needs a larger area or space for surfacing the detector in the electric device.
Moreover, the use of the luminous-efficiency correction filter increases the cost of the detector and the electric device.
It is therefore desirable to realize a photosensor having spectral responses closer to standard eye responses without the need of a luminous-efficiency correction filter.